1. Field of the Invention
The invention relates in general to an antenna module, and more particularly to an antenna module disposed on a printed circuit hoard.
2. Description of the Related Art
As technology progresses, people are getting more and more convenient in daily life. In terms of communications between people, innovative communication products provide closer connections between people. Among these products, mobile telephones are the most popular and have been standard devices for personal communication. For a few years, a short-range radio technology, called Bluetooth, is being developed and promoted, and is aimed at simplifying communications among mobile computers, mobile phones and other portable handheld devices, and connectivity to the Internet, thus allowing a more concrete wireless network structure. From the experiment of the development of wireless communication technology, wireless links between devices wilt be substituted for the use of cabling gradually.
A wireless circuit typically includes at least a main function circuit and an antenna module. The main function circuit is used for processing signals while the antenna module is used for signal transmitting and receiving. For the requirement of miniaturization, the main function circuit and the antenna module are required to be integrated on a single chip. This integrated circuit is expected to have a specific performance. In addition, it is expected to have a reduced production cost without affecting its performance.
FIG. 1A illustrates a dipole antenna 100. The dipole antenna 100 has two electrodes, electrodes 110 and 150, and the lengths of the two electrodes are one fourth of the wavelength of an excitation signal. In order to radiate signals from the dipole antenna 100, as it is excited, signals fed into the electrodes 110 and 150 are required to be reversed in phase. That is, the signals at terminals F1 and F2 have a phase difference of 180°. In practice, for providing the signals on the two electrodes with the phase difference of 180°, different signal paths for the two electrodes can be employed, resulting in a phase delay of 180°. The following description will provide an illustration for implementation in this way.
FIG. 1B illustrates a dipole antenna, where the excitation of the antenna is achieved with a single-ended input. As shown in FIG. 1B, when an excited signal is fed though a feed-pin F, the excited signal fed to the electrode 110 is along a path greater than the path that the excited signal is fed to the electrode 150 by the difference between L1 and L2, that is, by (L1-L2), For an electromagnetic wave, multiplication of its frequency and wavelength is equal to a constant C, that is, C=ƒλ. Thus, if the signal fed into the antenna has a frequency of ƒ and L1-L2=C/2ƒ, the phase difference between the signals fed into the two respective electrodes is of one half of the wavelength, that is, a phase difference of 180°, thus fulfilling the condition for radiation excitation.
A dipole antenna produced by a coaxial cable is illustrated in FIG. 2A. For making the dipole antenna, one can peel off a portion of the isolation layer and earth line from a coaxial cable 200 so that the core of the coaxial cable is exposed and acts as an electrode 210. The coaxial cable 200 is then covered with and coupled to a conductive casing as shown in FIG. 2A. The conductive casing is utilized as an electrode 250. It is obvious that the structure shown in FIG. 2A becomes a simple dipole antenna if the electrodes 210 and 250 are designed to have lengths of ¼ of the excited signal's wavelength. Referring to FIG. 2B, a cross-sectional view of the antenna taken along the line 2B—2B in FIG. 2A is shown, wherein the condition for being a dipole antenna is satisfied, As seen from FIG, 2B, the lengths of the electrodes 210 and 250 are equal to λ/4. During the antenna excitation, the excited signal can be fed into the core, and the earth line 290 can be grounded. Since the signals fed into the core and the earth line 290 have a phase difference of 180°, the signals fed into the electrodes 210 and 250 have a phase difference of 180°. Due to its simplicity, this method is widely used in the design of wireless circuits. Most of the mobile phones currently are designed by this method.
Before being radiated from the antenna, the signal is amplified by a power amplifier (PA). In addition, for the enhancement of the power amplifier's immunity to noise and common mode signal, the output signals of the power amplifier are designed as differential signals. That is, the power amplifier has a positive output terminal and a negative output terminal, and the magnitude of the output signal of the power amplifier is equal to the difference between the signals from the positive and negative output terminals. It should be noted that if the power amplifier outputs differential type signals, the output signals from the power amplifier must be converted into a single-ended signal when the antenna is required to be excited in a single-ended manner. A structure of a conventional antenna module for signal transmission is illustrated in FIG. 3A. The antenna module has a differential type power amplifier 320 with a positive output terminal P and a negative output terminal N, wherein the positive and negative output terminals form a differential pair. Since the antenna module is made with a coaxial cable, which is conventionally used in the industry, the output signals of the differential type power amplifier 320 are required to be converted into a single-ended signal before being fed into an antenna 370. In practice, a transformer 320 is connected between the differential type power amplifier and the antenna, for converting the differential type output signals into the single-ended signal. The transformer for this purpose is called a balun and is widely used in the industry.
On the other hand, the signal received by the antenna is a weak signal so that the received signal, before further processing, needs to be amplified by a low noise amplifier (LNA). Likewise, for the enhancement of the LNA's immunity to noise, the inputs of the LNA are designed as a differential pair. Thus, signal conversion is required to be concerned in the design of a receiving antenna. FIG. 3B shows a structure of a conventional receiving antenna module. As shown in FIG. 3B, the signal received by the antenna 370 is first converted into differential signals by the transformer 330. Next, the differential signals are fed into the LNA 340 for amplification. The LNA 340 has a positive input terminal P and a negative input terminal N. These two terminals have a phase difference of 180°, forming a differential pair to avoid interference.
For the conventional approaches described above, both the power amplifier and low noise amplifier are of differential type Thus, a transformer must be used when one of them is coupled to an antenna. From the viewpoint of a producer, the use of the transformer will unavoidably increase the production cost and reduce the product competitiveness. In addition, the transformer consumes power, thus affecting the efficiency. On the other hand, the dipole antenna made by the coaxial cable requires accurate in the length of the dipole antenna so as to achieve impedance matching. The process for achieving this is time-consuming. Besides, the coaxial-cable-made dipole antenna is not filly adaptable to a printed circuit board for circuit integration, thus giving little contribution to the circuit miniaturization.